The invention relates to membrane electrode assemblies and electrochemical cells such as fuel cells, electrolyzers and electrochemical reactors.
Fuel cells involve the electrochemical oxidation of a fuel and reduction of an oxidizing agent to produce an electrical current. The two chemical reactants, i.e., the fuel and the oxidizing agent, undergo redox reaction at two isolated electrodes, each containing a catalyst in contact with an electrolyte. An ion conduction element is located between the electrodes to prevent direct reaction of the two reactants and to conduct ions. Current collectors interface with the electrodes. The current collectors are porous so that reactants can reach the catalyst sites.
Fuel cells produce current as long as fuel and oxidant are supplied. If H2 is the fuel, only heat and water are byproducts of the redox reactions in the fuel cell. Fuel cells have application wherever electricity generation is required. Furthermore, fuel cells are environmentally benign.
An electrolyzer involves the splitting of water into hydrogen and oxygen using electricity. Similarly, an electrochemical reactor, such as a chlor-alkali cell, uses electricity to produce chlorine from an alkaline brine. Electrolyzers and electrochemical reactors basically involve a fuel cell operating in reverse. For example, for an electrolyzer to produce hydrogen and oxygen from water by passing an electrical current through the device, an equivalent ion conductive element appropriate for use in a fuel cell may be located between catalyst layers and current collector layers.
In a first aspect, the invention features an electrochemical MEA comprising:
an ion conductive membrane, the membrane having a first and second major surface;
catalyst adjacent to the first and second major surfaces; and
a porous, electrically conductive polymer film adjacent to the ion conductive membrane, the film comprising a polymer matrix and about 45 to about 98 percent by weight electrically conductive particles embedded within the polymer matrix.
In a preferred embodiment, the Gurley value of the polymer film is less than about 50 s/50 cc. The polymer matrix can include a polymer selected from the group consisting of polyethylene, polypropylene, polyvinylidene fluoride, polytetrafluoroethylene, poly(tetrafluoroethylene-co-perfluoro-(propyl vinyl ether)) and mixtures thereof. The electrically conductive particles can comprise carbon. The porous polymer film preferably has an electrical resistivity of less than about 20 ohm-cm.
The catalytic material can be disposed at an interface between the ion conductive membrane and the porous, electrically conductive polymer film. The catalytic material can be disposed upon the surfaces of the ion conductive membrane. In preferred embodiments, the catalytic material is disposed in nanostructured elements.
In another aspect, the invention features an electrochemical MEA comprising:
an ion conductive membrane, the membrane having a first and second major surface;
catalyst adjacent to the first and second major surfaces; and
a porous, electrically conductive polymer film adjacent to the ion conductive membrane, the film comprising electrically conductive particles and a porous matrix of fibrillated PTFE fibrils.
The catalytic material can be disposed at an interface between the ion exchange membrane and the porous, electrically conductive polymer film. The catalytic material can be disposed upon at least one major surface of the electrically conductive polymer film. The conductive particles can comprise carbon. The porous polymer film preferably has a Gurley value of less than 50 s/50 cc and an electrical resistivity of less than 20 ohm-cm.
In another aspect, the invention features a method of producing an electrically conductive polymer film comprising the step of heating a porous, polymer film comprising a polymer matrix and about 45 to about 98 percent by weight electrically conductive particles to a temperature within 20xc2x0 C. of the melting point of the polymer matrix for sufficient time to decrease the Gurley value of the film by at least about 25 percent and decrease the electrical resistivity of the film by at least about 25 percent while substantially maintaining the physical integrity and mechanical properties of the film upon cooling. The polymer matrix can include a polymer selected from the group consisting of polyethylene, polypropylene, polyvinylidene fluoride, poly(tetrafluoroethylene-co-perfluoro-(propyl vinyl ether)) and mixtures thereof. The conductive particles can comprise carbon and/or one or more conductive metals. The porous film preferably includes between about 80 and about 98 percent by weight conductive particles. The temperature can range between about 5 to about 20 degree centigrade above the melting temperature. The Gurley value of the film following heating preferably is less than 50 s/50 cc. The method can further comprise the step of using differential cooling for quenching the extruded film to create an asymmetric film with one side being denser and having smaller pores and the other side being less dense and having larger pores. The differential cooling can be accomplished through the use of a casting wheel at a controlled temperature.
In another aspect, the invention features a method of forming an electrode backing layer for an electrochemical MEA comprising the steps of:
(a) forming a polymeric film comprising a crystallizable polyolefin polymer matrix, conductive particles and a diluent for the polymer;
(b) applying surface texture to the polymeric film; and
(c) removing the oil before or after applying the surface texture.
In another aspect, the invention features a method of forming an electrochemical MEA comprising the step of placing an electrode backing layer on both sides of a polymeric ion conductive membrane, the electrode backing layers each comprising a gas permeable, electrically conductive porous film prepared as described in the preceding paragraph, wherein a catalyst layer is disposed between each of the ion conductive membrane and the electrode backing layers.
In another aspect, the invention features a method of forming an electrochemical MEA comprising the step of placing an electrode backing layer on both sides of a polymeric ion conductive membrane, the electrode backing layers each comprising a gas permeable, electrically conductive porous fibrillated PTFE film and conductive particles embedded in the film, wherein a catalyst layer is disposed between each of the ion conductive membrane and the electrode backing layers.
In another aspect, the invention features a method of producing a plurality of 5-layer MEAs, comprising the step of applying catalyst layers and electrode backing layers at suitable locations along a web of ion conduction membrane such that a plurality of 5-layer MEAs can be cut from the web of ion conduction membrane.
In another aspect, the invention features a film comprising greater than about 45 percent by weight conducting particles, the film having a surface exhibiting under contact with water a receding and advancing contact angles greater than 90xc2x0, wherein the advancing contact angle is no more than 50xc2x0 greater than the receding contact angle. The advancing contact angle preferably is no more than 30xc2x0 greater than the receding contact angle, more preferably no more than 20xc2x0 greater than the receding contact angle.
In another aspect, the invention features a method of producing a film comprising a polymer and greater than about 45 percent by weight conducting particles, the method comprising the steps of heating to a temperature from about the melting point to about 20 degrees C. above the melting point and then stretching the film from about 25 percent to about 150 percent of their original length.
In another aspect, the invention features a polymer web including a plurality of MEA elements. The MEA elements can be disposed along a continuous web of ion conducting polymeric material. The polymer web can further include nanostructured catalyst layers and/or suitably located seal material.
Electrode backing layers as described herein have high electrical conductivity, high gas permeability, good water management characteristics and significant production advantages. Membrane electrode assemblies (MEAs) incorporating the electrode backing layers can have improved performance as determined by the current produced at a given fuel cell voltage. Advantageously, films of the present invention exhibit adequate hydrophobicity for effective water management without incurring the expense or the need for a fluoropolymer coating, whose properties can change with use. The porous polymeric, electrode backing layers can be used in efficient, commercial production methods of multilayer MEAs including continuous roll processes. Continuous roll processing allows for the cost effective assembly of hundreds of electrochemical cell components at a relatively rapid rate.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims.